1. Field of the Invention
The present invention relates to a process and apparatus for production of H.sub.2 or CO by the partial oxidation of hydrocarbons, preferably methane. In particular the partial oxidation is carried out in the presence of water or carbon dioxide under water-gas shift reaction conditions. The invention was made with government support under DOE Grant No. DE-FG02-88ER13878-A02. The government has certain rights in the invention.
2. Related Art
Catalytic steam reforming of methane is currently the main industrial process to produce synthesis gas (CO and H.sub.2). The steam reforming reaction may be represented as: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
Reforming is highly endothermic, requiring energy input and also requiring contact times on the order of seconds. The resultant high H.sub.2 /CO ratio is also unsuitable for methanol and Fischer-Tropsch synthesis. Therefore additional downstream reactors are usually required to adjust this ratio by water-gas shift reaction at contact time of .apprxeq.1 second as: EQU CO+H.sub.2 O+H.sub.2 +CO.sub.2
Partial oxidation, on the other hand, is an exothermic reaction which can be represented by the reaction of methane with oxygen as follows: EQU CH.sub.4 +1/2O.sub.2.fwdarw.CO+2H.sub.2
To produce synthesis gas by steam reforming, high temperature heat input is primarily required at two process steps. First, sufficient steam at a high temperature and high pressure must be generated for mixing with the hydrocarbon feedstock and, second, the steam reforming of the steam and hydrocarbon mixture must take place at relatively high temperatures and pressures through a bed of solid catalyst. The equipment needed for these two heat transfers at high temperature and high pressure is necessarily quite expensive. The equipment for the steam reforming step is also costly because it must be adapted to permit the changing of the solid catalyst when that catalyst is spent or poisoned. Heat sources appropriate for the above two process steps are typically provided by fired heaters at high, continuing utility costs, also with high fluegas NO.sub.x production consequential to the high temperatures required in the furnace firebox.
The production of synthesis gas by partial oxidation is considered a desirable alternative to steam reforming since it overcomes some of the problems of steam reforming, see for example PCT publication WO 90/06282 and WO 90/06297. U.S. Pat. No. 4,844,837 to Heck et al discloses a catalytic partial oxidation method for methane using a monolith catalyst with platinum-palladium, palladium-rhodium, or platinum-rhodium coatings. U.S. Pat. No. 4,087,259 to Fujitani et al describes a monolith catalyst with a rhodium coating to perform catalytic partial oxidation on gasoline and heavier petroleum fractions. U.S. Pat. No. 5,648,582 to Schmidt et al discloses the partial oxidation of methane at short residence times using metal deposited on a ceramic monolith.
In these conventional partial oxidations, methane (natural gas) is converted to high purity H.sub.2 and CO with a mole ratio of H.sub.2 :CO .sub..apprxeq. 2.0, which is the desired feed ratio for methanol and Fischer-Tropsch plants. However, many other applications require different ratios of H.sub.2 :CO. The present synthesis gas shift reactor modifies the prior processes to obtain either high purity H.sub.2 or CO.
Recent advancements in fuel cell technology have spurred an interest in converting natural gas into hydrogen. Pure hydrogen streams can be produced by steam reforming followed by high temperature shift using an Fe based catalyst, and low temperature shift using a Cu based catalyst. For natural gas to be an effective H.sub.2 source for fuel cells, the present natural gas conversion technology must be simplified, preferably to a single, highly selective small catalytic reactor. Other applications include acetic acid production, which requires pure CO feeds. We have discovered that the H.sub.2 :CO product ratio can be altered by the addition of CO.sub.2 or H.sub.2 O through the water-gas shift reaction while the conversion of CH.sub.4 remains constant, indicating negligible reforming is occurring.